On the cover: Scanning electron microscopy of spread Cdc42-deficient B cells activated for 2 d with anti-CD40 + IL-4 and incubated on anti-CD44–coated cultures. Note the lack of long protrusions and the presence of brushlike extensions. Pseudo-coloring is used to highlight the cells. Gerasimcik, N., C. I. M. Dahlberg, M. A. P. Baptista, M. J. Massaad, R. S. Geha, L. S. Westerberg, and E. Severinson. 2015. The Rho GTPase Cdc42 is essential for the activation and function of mature B cells. J. Immunol. 194: 4750–4758.